A number of different commercial flow meters are available on the market for the measurement of the water content of oil. Some meters are based on the use of radioactive radiation, some are capacitive, and some are based on the use of microwaves.
Microwave sensors are attractive because they are not limited by the health risks associated with radioactive radiation based meters and their fairly low accuracy or the undesirable influence of contamination on the capacitive sensors.
An example of a method for measuring properties of flowing fluids and a metering device and a sensor used for performing this method has been described in International Patent Application PCT/NO01/00200, for which a US-patent has been granted (U.S. Pat. No. 6,826,964 B2). The sensor uses the microwave resonance principle for the measurement of oil-continuous fluids (water drops and gas bubbles in oil, i.e. the oil is a continuous phase) and the measurement of conductivity for water-continuous fluids (oil drops and gas bubbles in water, i.e. the water is the continuous phase, and is intended for installation at a production zone inside an oil well.
Another example of a method for measuring flowing fluids with a far higher gas content, i.e. wet gas (a wet gas flow is a multiphase flow with a high gas volume fraction, usually called the gas void fraction (GVF), typically >99%) or high-gas multiphase flow, has been described in U.S. Pat. No. 6,915,707. This is also based on the microwave resonance principle.
The microwave resonance principle is based on measuring the permittivity/dielectric constant of the flow. Because the permittivity of water is high (in the order of 80) compared to that of oil (in the order of 1.5-3) or gas (even lower than for oil) the permittivity of a mixture of these three constituents is dominated by the contribution from water. Methods based on measuring the permittivity (microwave and capacitive methods) therefore provide the highest sensitivity for measuring the WVF of a mixture. Because microwave resonators are inherently stabile and the resonant frequency and quality factor (Q-factor), which are the two measurable properties of a resonance, can be measured with a high accuracy, the microwave resonance method is the most sensitive and accurate method available for measuring the WVF of a wet gas flow. However, when the WVF becomes very low, the permittivity of the mixture (i.e. the flowing fluid) starts to become dominated by the contributions from the oil and the gas. Especially the permittivity of the gas depends on the pressure and the temperature. To be able to resolve the contribution from the water one needs to know the contributions from the gas and the oil. E.g. the invention described in U.S. Pat. No. 6,915,707 also uses the hydrocarbon composition and measurements of temperature and pressure as inputs, and models for calculating the permittivity of the oil and the gas. The accuracy of the measurement of the WVF is then limited by the accuracy of the models, and the accuracy of the measurements of the temperature and pressure.
In a wet gas flow it is important to know the WVF because of the problems with hydrate formation, scaling, and corrosion caused by the water. Also the salinity of the water, caused by the production of formation water, is a very important factor as it strongly affects both corrosion and the formation of scale. As described above, present measurement solutions have limitations in the low end of WVF, while the known problems of hydrate formation and corrosion are still significant. Although at low WVF it will take more time for the smaller amount of water (possibly formation water) to cause deteriorating effects on the production flow, the problems are still highly relevant. Thus, it would be desirable to be able to improve the quantitative measurement of WVF in the flow, particularly at low water volume fractions, as the uncertainty of presently commercially available meters is limited at low values of WVF.
In this specification the Q-factor is defined as 2π multiplied with the stored energy/the loss in one oscillation period. Transferred to measurable variables this means that the Q-factor is obtained through the ratio between the resonance frequency and the peak width. The peak width is measured 3 dB beneath the top, i.e. the half effect width. According to a preferred embodiment of the invention both the Q-factor are measured, thus enabling the calculation of both the salinity and the water content.
The object of the invention is achieved by providing a method of measuring a wet gas flow in a pipeline according to the invention and a corresponding measuring instrument for performing the method according to the invention. More specifically the object of the invention is obtained as disclosed in the accompanying independent claims.